This invention relates to a system for generating and detecting telemetry pulses on electrical conductors, such as power lines.
Telemetry systems have been provided in the past for generating electrical pulses at one location on electrical conductors and for detecting the pulses at another position, in order to convey information between the two locations. For example, the Woods U.S. Pat. No. 3,594,584 and the Woods et al U.S. Pat. No. 3,815,006 show such systems wherein the conductors are power lines between an AC power supply and a load. As an example, the pulse generator or transmitter may be responsive to a condition (such as excessive temperature) of the load and generate pulses when the condition exists, and a pulse receiver or detector may operate a device such as an indicator or a contactor which opens the power lines to an overheated load.
A problem which has been encountered with prior art systems of the foregoing character is that they are susceptible to error caused by noise pulses which frequently appear on the power lines. The receiver has not been able to distinguish between the transmitter pulses and the noise pulses, and as a consequence the system has been subject to false tripping.
There are two general types of noise signals on power lines. One type may be called random noise pulses which appear essentially randomly and continuously. The other type may be referred to as synchronized pulses because they appear in equal numbers and at regular locations in both halves of the line voltage waveshape. The latter type of noise may be produced by SCR motor drives, light dimmers, etc.
It is a general object of the present invention to provide an improved telemetry system which is substantially immune to noise pulses on the power lines.